Just found these in Car & Driver's May05 instrument testing issue. Appologies if this is duplicate - I didn't see anything in the stickies.



"One of the best uses for these performance meters is to track the effectiveness of modifications done on a car. First you test the car in stock condition and record the results. Then you install a new exhaust system, or whatever, and test it again.

But there's a problem: A car will run faster on a cool day than it will on a hot day because cooler air is denser and contains more oxygen, allowing the engine to burn more fuel. Similarly, high barometric pressure produces more power than low pressure, and dry air has more oxygen than moist air. So how do you know if a quicker run was due to the new exhaust or more favorable weather?

Because we conduct much of our testing in Michigan, with its extremes of bitter cold and withering heat, we are always dealing with this problem. To eliminate the effects of weather we use a mathematical calculation to adjust our test results to dry air at sea level (barometric pressure of 14.7 psi) and a temperature of 60 degrees Fahrenheit. If it's cold out, the weather correction typically slows down our raw results, and conversely, when it's hot, the performance results improve.

In summary, we always measure the prevailing wet- and dry-bulb temperatures and absolute barometric pressure at each of our test sessions. We use these data to calculate the amount of dry air present and refer to a series of charts to adjust our raw performance numbers to compensate for these conditions."

http://www.caranddriver.com/article....rticle_id=9382

And found this nice site for the vapor pressure
http://www.csgnetwork.com/vaporpressurecalc.html

 

Currently I am using a Davis Vantage Pro II weather station. http://www.davisnet.com/weather/products/stations.asp

This is coupled to a software package from Family Software called RaceLog Pro 4. http://www.ifamilysoftware.com/rlp.html

It is basically an ET Predictor program that will tell you what to dial based on how the weather has changed since your last run.

It also corrects to a weather standard so you can compare runs as though they were all done under the same weather conditions so you can actually see where or if there are really differences or were they just changing weather conditions.

Expensive, but I believe really worth it if you race a lot. Yesterday I won the Street Eliminator class against a field of V8's mostly running 12's. My winning slip I dialed a 13.85 and ran a 13.850. Thats what technology can do, however your car has to be running consistently before this will help you to actually make fair comparisons.

 

The easiest way is to convert to density altitude (DA). That's what the pro's use.

1) Find your historical weather conditions here:

www.wunderground.com

2) Enter in the conditions here:

http://wahiduddin.net/calc/calc_hp_dp.htm

3) Correct ET/MPH here:

http://www.gnttype.org/techarea/misc/altitude.html